Abstract
High-grade serous ovarian cancer (HGSOC) is a poor prognostic disease, especially in BRCA1/2 wild-type (BRCA-WT) patients with homologous recombination (HR) proficiency. These patients often show limited response to both platinum-based chemotherapy and PARP inhibitors. HR and non-homologous end joining (NHEJ) are the two major DNA double-strand break (DSB) repair pathways. HR is a precise repair mechanism for DSBs but is limited to S and G2 phases. In contrast, NHEJ functions more broadly throughout the cell cycle, including G1. We investigated whether inhibiting the G2/M checkpoint kinases PLK1 or WEE1 individually could disrupt mitotic control and expose therapeutic vulnerabilities in BRCA-WT/HR-proficient HGSOC cells. We evaluated cell cycle-targeted strategies to overcome HR-proficient chemoresistance using either volasertib (a selective PLK1 inhibitor) or adavosertib (a potent WEE1 inhibitor) in BRCA-WT/HR-proficient and BRCA-mutant/HR-deficient HGSOC models. Both agents induced DNA damage, impaired HR repair (reduced RAD51 foci), and triggered mitotic catastrophe-a form of cell death caused by defective mitosis and unresolved DNA damage-in BRCA-WT cells. Volasertib caused polyploidy and abnormal spindle formation, indicating mitotic slippage and cytokinesis failure, whereas adavosertib abrogated the G2/M checkpoint, forcing premature mitotic entry. In contrast, BRCA-mutant cells were resistant to either volasertib or adavosertib, consistent with sustained and functional NHEJ activity. This resistance was restored by the pharmacological or genetic inhibition of DNA-PKcs (DNA-dependent protein kinase, catalytic subunit), a prominent component of NHEJ. Functional and xenograft models confirmed selective vulnerability of BRCA-WT tumors to either PLK1 or WEE1 inhibition. Our work highlights a mechanistic framework linking cell cycle checkpoint inhibition to DNA repair pathway selectivity, providing a rationale for targeting mitotic regulators in HR-proficient ovarian cancer-a subgroup with high clinical unmet need.